Confluence guests (Confluence guest access): Supported. People you invite as guests to a Confluence space can open pages that contain the app's macros, just like any other Confluence user. The SharePoint content they see still follows their own SharePoint permissions.
Microsoft 365 guest users in SharePoint (external users): Currently not supported. External (guest) users of your SharePoint or Microsoft 365 tenant cannot access embedded SharePoint content through the app. This worked while the app ran on Atlassian Connect, but since it moved to Atlassian Forge, it no longer works: Forge uses a fixed OAuth2 path that cannot be resolved per tenant, so guest users, who belong to their own tenant, cannot be matched to the app operator's tenant.